1. Field of the Invention
This invention relates to the field of switching voltage regulators, and particularly to multi-phase switching voltage regulators.
2. Description of the Related Art
Switching voltage regulators provide a predetermined and substantially constant output voltage from a source voltage that may be fluctuating, or that may be at an inappropriate amplitude for the load. Such regulators typically employ one or more switching elements. The switching elements may be, for example, field-effect transistor (FET) switches. Control circuitry regulates the current supplied to the load by varying the ON-OFF times of the switching elements (i.e., the regulator""s duty cycle, which is the percentage of time that a switch is ON during a cycle of operation). Inductors and capacitors are typically used to convert the switched current pulses into a steady flow of load current. Switching regulators are frequently used in portable battery-powered electronic products, as they provide high operating efficiency and long battery life with little heat generation.
A switching voltage regulator may be arranged to operate using one or more different regulation modes. For example, for voltage-mode regulation, a feedback signal Vfb which varies with the regulator""s output voltage is fed to a comparator along with a periodic sawtooth waveform; the comparator""s output controls the duty cycle of the switching elements. For current-mode regulation, a voltage Vi is generated which is proportional to the current in the output inductor. Vi and Vfb are provided as inputs to a comparator, the output of which controls the duty cycle of the switching elements.
Several methods have been employed to obtain inductor current information for use in a switching regulator""s control loop. Typically, a small voltage is sensed, either continuously or periodically sampled, to determine the current. One approach is described in U.S. Pat. No. 5,982,160 to Walters et al., in which an R-C network is connected across the output inductor, with the small resulting signal sent to the controller. Another approach is described in U.S. Pat. No. 5,847554 to Wilcox et al., which samples the voltage drop across the regulator""s MOSFET switches when they are on.
Advanced microprocessors require power supplies that provide a low voltage (e.g.,  less than 2 volts) at high current. One way in which high currents are provided is with a multi-phase switching regulator, in which the components of a number (N) of single-phase regulators are repeated to produce N output currents, which are summed together to provide the total output current. However, implementing any of the regulators cited above as a multi-phase regulator requires summing and/or sampling data in the controller IC, which can introduce unwanted complexity and processing delays into the IC design.
Another switching regulator issue concerns maintaining the output voltage within specified limits in response to load transients. One approach to this problem is known as xe2x80x9cadaptive voltage positioningxe2x80x9d (AVP), in which the output voltage is positioned within the specified range so that it best withstands a load transient. This is conventionally accomplished by establishing a xe2x80x9cdroop impedancexe2x80x9d for the output, by inserting information about the desired output current droop into the output voltage feedback loop as an error signal. However, this approach can result in sluggish and imprecise control on the output voltage for dynamic changes in the load.
A multi-phase switching voltage regulator is presented which overcomes the problems noted above, enabling precise multiple-loop control which is particularly well suited to AVP power supplies.
The invention provides a xe2x80x9ctotal current circuitxe2x80x9d, which makes use of an amplifier circuit to sum together individual phase currents to generate a signal representing the total instantaneous output current in the inductors Iout. For an N-phase switching regulator, N current sensing elementsxe2x80x94which can be the output inductors themselves or current sense resistors connected in series with the inductorsxe2x80x94carry respective phase currents. A summing circuit, preferably made from N resistors each having a resistance Rp, is connected to sum the voltages present at the switch node sides of the sensing elements, and to provide the summed voltages at a summing node. An amplifier circuit is connected to the summing node at a first input and to the regulator""s output terminal at a second input, and produces an output voltage Vcs. When Vout is subtracted from Vcs, the result is proportional to Iout.
The amplifier circuit preferably comprises an operational amplifier, with a feedback resistor having a resistance Rcs, and a filter capacitor having a capacitance Ccs connected across the feedback resistor. The op amp is arranged such that, when the time constant RcsCcs is made substantially equal to the inductors"" time constant L/R1, the output voltage Vcs of the op amp is given by:             V      cs        =                  V        out            -                                                  R              l                        ⁢                          R              cs                                            R            p                          ⁢                  I          out                      ,
where R1 is the inductors"" resistance and Vout is the regulator""s output voltage. Vcs is thus proportional to total instantaneous inductor output current Iout.
The invention also provides a unique means for direct insertion of total inductor output current information into a switching regulator""s voltage-mode control loop, to provide AVP for the output voltage. A desired output voltage is specified by providing a reference voltage Vref, and a current circuitxe2x80x94preferably the total current circuit described abovexe2x80x94produces an output voltage Vdroop which varies with the the regulator""s total filtered instantaneous output current Iout(fltr). A summation circuit produces an output voltage Vset which is given by:
Vset=Vref+Vdroop. 
Vset and Vout are presented to the inputs of a voltage control error amplifier circuit, which produces an output Verr that varies with the difference between Vset and Vout. A control circuit operates the regulator""s switches in response to Verr. The control circuit, total current circuit, summation circuit, and voltage control error amplifier circuit are arranged such that the regulator""s output has a desired droop impedance Ro, to provide AVP of Vout as a function of the total filtered instantaneous inductor output current Iout(fltr). This serves to directly insert instantaneous inductor output current information into the reference voltage input of the regulator""s voltage control error amplifier, which enables fast, accurate control of output voltage for dynamic changes in load.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.